Blog Client with Factory and Validator

This implements a simple initial blog client that covers the basic blog
actions, but no deletion/removal of blogs, yet.

Classes for Blogs and Blog Post Headers have been introduced along with
the associated factories.

A `BlogPostValidator` has been added that validates incoming blog posts.

Closes #402
Closes #404

briar-api/src/org/briarproject/api/blogs/BlogPostHeader.java

@@ -0,0 +1,43 @@
+package org.briarproject.api.blogs;
+
+import org.briarproject.api.clients.PostHeader;
+import org.briarproject.api.identity.Author;
+import org.briarproject.api.identity.Author.Status;
+import org.briarproject.api.sync.MessageId;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public class BlogPostHeader extends PostHeader {
+
+	@Nullable
+	private final String title;
+	@NotNull
+	private final String teaser;
+	private final boolean hasBody;
+
+	public BlogPostHeader(@Nullable String title, @NotNull String teaser,
+			boolean hasBody, @NotNull MessageId id,
+			@Nullable MessageId parentId, long timestamp,
+			@NotNull Author author,	@NotNull Status authorStatus,
+			@NotNull String contentType, boolean read) {
+		super(id, parentId, timestamp, author, authorStatus, contentType, read);
+
+		this.title = title;
+		this.teaser = teaser;
+		this.hasBody = hasBody;
+	}
+
+	@Nullable
+	public String getTitle() {
+		return title;
+	}
+
+	@NotNull
+	public String getTeaser() {
+		return teaser;
+	}
+
+	public boolean hasBody() {
+		return hasBody;
+	}
+}